In an electrophotographic image forming apparatus such as a laser printer, an electrostatic copying machine, a plain paper facsimile machine or a printer-copier-facsimile multifunction machine, a nonmagnetic single-component developing system is mainly employed as a developing system.
In the nonmagnetic single-component developing system, toner is passed through between a developing roller and a toner amount regulating blade to be triboelectrically charged and carried on a surface of the developing roller, whereby a toner layer is formed on the surface. Then, the toner layer thus formed is brought into direct contact with a surface of a photoreceptor body formed with an electrostatic latent image, whereby the toner is selectively transferred from the toner layer to the electrostatic latent image to develop the electrostatic latent image into a toner image. Alternatively, the toner layer is brought into noncontact adjacent relation to the surface of the photoreceptor body, whereby the toner is selectively transferred (or allowed to selectively jump) from the toner layer to the electrostatic latent image to develop the electrostatic latent image into a toner image.
The developing roller typically includes a roller body of a nonporous single-layer structure formed by preparing a semiconductive rubber composition containing rubber, a crosslinking component for crosslinking the rubber and an electron conductive agent and/or an ion conductive agent, forming the semiconductive rubber composition into a tubular body, and crosslinking the rubber of the rubber composition of the tubular body.
However, the roller body of the conventional developing roller described above has a relatively high hardness, so that the toner is liable to be frictionally broken to be thereby deteriorated. This may result in formation of coarse images. Further, fine particles of an external additive are liable to be frictionally buried in toner particles to thereby impair the fluidity of the toner. This may result in formation of uneven-density images or in formation of streaked uneven-density images due to a so-called slip-stick phenomenon of the toner amount regulating blade. Therefore, the roller body is required to be highly flexible for prevention of these imaging defects.
In Patent Document 1, it is contemplated to use liquid silicone rubber as the rubber to improve the flexibility of the roller body. As described in Patent Document 1, however, the liquid silicone rubber should be cast-molded into a roller body shape with the use of a mold conformal to the outer shape of the roller body. This requires complicated operations, resulting in reduction in productivity.
As in the conventional case, it is also conceivable to prepare a soft rubber composition by blending a crosslinking component and oil as a softening agent with solid rubber which can be formed into a predetermined shape by an extrusion method or the like without the use of the mold, and form the soft rubber composition into a flexible roller body. However, the oil is liable to bleed on the outer peripheral surface of the roller body to result in contamination of the photoreceptor body and the like.
To cope with this, it is contemplated to prepare a rubber composition by blending the solid rubber, the crosslinking component and a foaming component for foaming the rubber, and produce a flexible roller body of a porous structure from the rubber composition through a forming step, a crosslinking step and a foaming step.
In some case, opposite end portions of the outer peripheral surface of the roller body are respectively sealed with seal members in order to prevent the toner carried on the outer peripheral surface of the roller body from leaking outside a developing unit incorporating the developing roller. The seal members are formed, for example, of felt or the like, and fixed to a housing of the developing unit and kept in sliding contact with the opposite end portions of the outer peripheral surface of the roller body of the developing roller during rotation of the developing roller.
However, the toner is liable to leak from the opposite end portions of the roller body sealed with the seal members when image formation is repeatedly performed. This is because the opposite end portions of the outer peripheral surface of the roller body are liable to be worn due to the sliding contact with the seal members and gaps are formed between the opposite end portions and the seal members.
Particularly, where the roller body has the porous structure to be imparted with higher flexibility, as described above, the roller body is more liable to be worn. Therefore, the leakage of the toner and the like may occur in a shorter period of time. To cope with this, it is contemplated to coat at least the seal member contact portions of the outer peripheral surface with coating films to reduce the friction occurring between the seal member contact portions and the seal members for suppression of the friction, while imparting the roller body per se with a porous structure having proper flexibility (see, for example, Patent Document 2 and the like).
The coating film is formed by preparing a liquid coating agent containing a binder resin such as a urethane resin, a phenol resin, a fluororesin or a silicone resin, applying the liquid coating agent onto the outer peripheral surface of the roller body by a spraying method, a dipping method or other applying method, drying the applied coating agent and, optionally, curing the urethane resin or the phenol resin. Alternatively, as disclosed in Patent Document 1, the surface roughness of the surface of the coating film, i.e., the surface roughness of the outer peripheral surface of the roller body, may be adjusted by blending roughness imparting particles such as urethane particles in the coating agent to control the amount of the toner carried on the outer peripheral surface and the toner charge amount.
The coating film of the urethane resin or the phenol resin is excellent in wear resistance, but is less slidable. This increases the rotating torque of the developing roller, making proper image formation difficult. On the other hand, the coating film of the fluororesin or the silicone resin is excellent in slidability, but is insufficient in wear resistance. Therefore, the coating film is liable to be worn out in a shorter period of time to lose its effect.
During the formation of the coating film, the coating film is liable to suffer from various inconveniences, i.e., contamination with foreign matter such as dust and uneven thickness. For preparation of the liquid coating agent, an organic solvent is required for dissolving the binder resin. The use of the organic solvent may exert a great load on the environment, and go against a recent trend toward reduction of VOC (volatile organic compounds). In addition, the binder resin is limited to those soluble in the organic solvent.
It is also contemplated to form a porous tubular body as an inner layer from a semiconductive rubber composition and cover the generally entire outer peripheral surface of the inner layer with an outer layer of a seamless nonporous tube formed from a semiconductive resin to produce a roller body (Patent Documents 3, 4 and the like).
The tube is formed, for example, by extruding the resin. A resin excellent in strength, wear resistance and the like can be selected for use as the resin for the tube without the need for consideration of the solubility in the organic solvent. Therefore, the outer layer of the nonporous tube has higher strength and excellent wear resistance and, unlike the coating film formed from the liquid coating agent, is prevented from losing its effect due to the wear-out in a short period of time.